A complete analysis of the thermodynamics of the allosteric enzyme aspartate transcarbamylase will be made using flow microcalorimetry. The energetics of the interactions between its subunits, and of the binding substrates, inhibitors, and activators will be studied over a wide range of experimental conditions, using both the native enzyme and chemically modified or mutant enzymes with altered allosteric properties. Specifically, the following reactions will be investigated: 1. The binding of phosphate, carbamyl phosphate, phosphonacetamide and acetyl phosphate to the catalytic subunit and the intact enzyme. 2. The binding of L-aspartate, succinate, fumarate, and D- and L-malate to the catalytic subunit and the intact enzyme, in the presence of various phosphate-containing compounds. 3. The binding of CTP, BrCTP, and ATP to the regulatory subunit and the intact enzyme. 4. The dissociation of the intact enzyme into its catalytic and regulatory subunits in the presence of mercurials. 5. The association of the catalytic and regulatory subunits to form the intact enzyme in the presence of various effectors. 6. The acid-base titration of the intact enzyme and its subunits.